fix(todo-continuation): remove activity-based stagnation bypass

Activity signals (tool calls like compress, grep, bash) were treated as
'progress' by the stagnation detector, resetting the stagnation counter
every cycle. This prevented MAX_STAGNATION_COUNT from being reached,
causing infinite continuation loops when models degrade to minimal
responses in long sessions (e.g. GLM-5.1 at ~100K tokens).

Stagnation now only tracks actual todo state changes: incomplete count
decrease, completed count increase, or todo snapshot change. Tool-level
activity no longer resets the stagnation counter.

src/hooks/todo-continuation-enforcer/session-state.test.ts

@@ -144,9 +144,9 @@ describe("createSessionStateStore", () => {
     expect(stagnatedAgainUpdate.stagnationCount).toBe(1)
   })
 
-  test("given non-codex activity happens after a successful continuation, treats it as progress", () => {
+  test("given tool activity happens after a successful continuation without todo changes, keeps counting stagnation", () => {
     // given
-    const sessionID = "ses-non-codex-activity-progress"
+    const sessionID = "ses-activity-stagnation"
     const state = sessionStateStore.getState(sessionID)
     const todos = [
       { id: "1", content: "Task 1", status: "pending", priority: "high" },
@@ -154,40 +154,12 @@ describe("createSessionStateStore", () => {
 
     sessionStateStore.trackContinuationProgress(sessionID, 1, todos)
     state.awaitingPostInjectionProgressCheck = true
-    sessionStateStore.recordActivity(sessionID)
 
     // when
     const progressUpdate = sessionStateStore.trackContinuationProgress(
       sessionID,
       1,
       todos,
-      { allowActivityProgress: true },
-    )
-
-    // then
-    expect(progressUpdate.hasProgressed).toBe(true)
-    expect(progressUpdate.progressSource).toBe("activity")
-    expect(progressUpdate.stagnationCount).toBe(0)
-  })
-
-  test("given codex activity happens after a successful continuation, keeps counting stagnation", () => {
-    // given
-    const sessionID = "ses-codex-activity-stagnation"
-    const state = sessionStateStore.getState(sessionID)
-    const todos = [
-      { id: "1", content: "Task 1", status: "pending", priority: "high" },
-    ]
-
-    sessionStateStore.trackContinuationProgress(sessionID, 1, todos)
-    state.awaitingPostInjectionProgressCheck = true
-    sessionStateStore.recordActivity(sessionID)
-
-    // when
-    const progressUpdate = sessionStateStore.trackContinuationProgress(
-      sessionID,
-      1,
-      todos,
-      { allowActivityProgress: false },
     )
 
     // then